This invention relates to a wireless communication system, and more particularly, to a wireless communication system in which the same frequency as that of a primary system is used secondarily.
Generally speaking, a white space is a frequency band that is within a frequency band for which a wireless system given priority in using a frequency in question (a primary system) is defined in the Radiocommunication Sector, International Telecommunication Union (ITU-R) or the like, and that is not used by the primary system in terms of time or area. Studies for making effective use of white spaces are being conducted due to the shortage of frequencies suitable for wireless communication which results from the popularization of cellular phones and increased traffic.
Of this kind of frequency bands, a white space centered around a TV band is a TV white space. In other words, a TV white space is a frequency band that is within one of the VHF band and the UHF band, which are allocated to terrestrial TV broadcasting, and that is not used for TV broadcasting in terms of time or area. Sharing frequencies in this frequency band between a business operator who broadcasts a TV program and the like in this frequency band (a primary user) and other business operators or individuals (secondary users) on the condition that the use of the primary user be not affected is being discussed.
One of the standardization activities that take the above-mentioned movement into consideration is IEEE 802.22. IEEE 802.22 is targeted for Wireless Regional Area Networks (WRANs) that utilize TV white spaces. The UHF frequency band (470 MHz to 710 MHz) for TV is a frequency band lower than frequencies that are used by cellular phones (the 800 MHz band and the 2 GHz band), and its radio waves have a property of reaching far and wide. IEEE 802.22 therefore assumes the coverage of a single base station to be 20 km to 30 km. Of the layers of the Open Systems Interconnection (OSI) reference 7-layer model, IEEE 802.22 standardizes the physical layer and the Media Access Control (MAC) layer.
FIG. 21 illustrates the configuration of a wireless communication system which conforms to the draft specifications of IEEE 802.22.
A base station (BS) 101 and a customer premise equipment (CPE) 102 are coupled by a wireless method defined in IEEE 802.22, and the base station 101 is coupled to a network (for example, IP network) 103 via a cable line. In order to run this IEEE 802.22 system, a network control system 104, a network management system 105, and a database service 106 are coupled via the network 103. IEEE 802.22, which mainly defines the physical layer and the MAC layer between the base station 101 and the customer premise equipment 102, does not regulate the specifications of an interface to an external system or service that is coupled via the network 103.
Applying IEEE 802.22 to a TV frequency band in this case means that a TV broadcasting which is a primary system and the IEEE 802.22 system which is a secondary system share the frequency band, and inherently causes the “hidden node problem”.
The “hidden node problem” is described with reference to FIGS. 22(a) and 22(b). A TV signal transmitted from a TV station 201 reaches a TV receiver 202, with attenuation an amount of which is in relation to the distance. A receiving sensitivity necessary to receive a TV signal and displays video and the like is set to the TV receiver. A TV program can be viewed on the TV receiver in a case where the receiving power is equal to or more than the receiving sensitivity, and cannot be viewed in a case where the receiving power is less than the receiving sensitivity. FIG. 22(a) illustrates a state where the receiving power is equal to or more than the receiving sensitivity at the TV receiver 202, which is located at a distance from the TV station, and a TV program can therefore be viewed on the TV receiver 202 (204).
The IEEE 802.22 system is allowed to run as long as its operation does not affect the TV broadcasting which is the primary system. An IEEE 802.22 base station/customer premise equipment (BS/CPE) 203 therefore executes sensing of radio waves of TV broadcasting. In sensing, the IEEE 802.22 system needs to be capable of detecting a TV signal at a sensitivity higher than that of the TV receiver 202 (a TV signal of a lower receiving power). FIG. 22(a) illustrates an example in which the IEEE 802.22 base station/customer premise equipment 203 is located even farther than the TV receiver 202 from the TV station. A signal detected at the location of the IEEE 802.22 base station/customer premise equipment 203 in this case has a lower receiving power than the power at which a TV program can be viewed on the TV receiver 202 (205). Then it is determined that this frequency is not in use at the location of the IEEE 802.22 base station/customer premise equipment, and the frequency is used for communication between the IEEE 802.22 base station and the IEEE 802.22 customer premise equipment.
The IEEE 802.22 base station/customer premise equipment 203 consequently uses the same frequency as that of TV broadcasting to send out radio waves as illustrated in FIG. 22(b) (206). The IEEE 802.22 base station/customer premise equipment 203 at this point is not aware of the existence of the TV receiver 202, which is simply receiving radio waves from the TV station 201. As a result, the TV receiver 202 receives an unwanted IEEE 802.22 signal at a power higher than that of the wanted TV broadcasting signal (207). Interference from the unwanted IEEE 802.22 signal hinders the viewing of a TV program, which constitutes the “hidden node problem”. The TV receiver 202 is just a receiver and does not transmit radio waves of its own. The IEEE 802.22 base station/customer premise equipment 203 therefore cannot detect the TV receiver 202.
IEEE 802.22 is accordingly designed so that a TV white space network service area 303 does not overlap a TV broadcasting service area 301 by providing a keep out region 302 outside the TV broadcasting service area 301 and setting up a base station (101) outside the keep out region 302.
A procedure that the customer premise equipment 102 follows in order to couple to an IEEE 802.22 network is described next. FIG. 24 is a flowchart illustrating a procedure through which the customer premise equipment 102 couples to an IEEE 802.22 network.
The customer premise equipment 102 first conducts a self test (401) and then obtains antenna gain information (402). The customer premise equipment 102 next executes sensing for detecting a TV signal 403, or a signal 404 broadcast at a TV frequency in question from an IEEE 802.22 or a similar system (a WRAN broadcast signal) (405). Based on the result of the sensing 405, the customer premise equipment 102 chooses a WRAN network to which the customer premise equipment 102 is to be coupled (WRAN selection) (406). A GPS receiver of the customer premise equipment 102 obtains its own location (geolocation data acquisition) (407). The customer premise equipment 102 determines whether or not its own location has successfully been obtained (“Success?”) (408). In a case where succeeding in obtaining its own location, the customer premise equipment 102 obtains WRAN parameters, which are information for coupling to the base station 101 (for example, time information), from the WRAN broadcast signal 404 broadcast from the network side (WRAN parameters acquisition) (409). The customer premise equipment 102 adjusts the antenna azimuth (antenna azimuth adjustment) (410), and then transmits radio waves to the base station 101 in an attempt to couple to the base station 101 (initial ranging) (411). Thereafter, an authentication process for determining whether or not the customer premise equipment 102 is a legitimate node (an AAA authentication process) (412), and whether or not the customer premise equipment 102 has been authenticated successfully is determined (“Success?”) (413). In a case where the customer premise equipment 102 is authenticated successfully, registration is executed (414), and IP connectivity between the customer premise equipment 102 and the base station 101 is established (IP connectivity establishment) (415).
In the case where the customer premise equipment 102 is set up stationarily in the IEEE 802.22 system, in other words, in a case where the customer premise equipment 102 does not move from a setup location which can be known in advance, the customer premise equipment can be set up within the TV white space network service area 303 of FIG. 23. In the case where the customer premise equipment 102 is portable equipment and the place of its use can be moved, on the other hand, the customer premise equipment 102 may move out of the TV white space network service area 303 at the will of the user.
This problem is described with reference to drawings. In the case where the customer premise equipment 102 is set up outside the TV white space network service area 303, for example, as illustrated in FIG. 25, increasing the transmission power (Tx power) of the base station 101 in order to deliver the broadcast signal to the location of the customer premise equipment 102 expands the TV white space network service area (901). This creates an area 902 which overlaps the TV broadcasting service area. Interference affects a TV receiver that receives TV broadcasting inside the area 902, and gives rise to a problem of deteriorated TV reception quality.
Another example is described with reference to FIG. 26. In the case where the customer premise equipment 102 communicates a signal to the base station 101 at a Tx power enough to reach the base station 101 from the location of the customer premise equipment 102, an area 1002 is created, where a region 1001 at which a signal transmitted from the customer premise equipment 102 arrives overlaps the TV broadcasting service area 301. Interference affects a TV receiver that receives TV broadcasting inside the area 1002, and gives rise to a problem of deteriorated TV reception quality.
In the network coupling procedure of IEEE 802.22, where a WRAN network to which the customer premise equipment is to be coupled is chosen (406) based on the result of sensing (405) as illustrated in FIG. 24, WRAN system selection cannot be executed in a case where the customer premise equipment 102 moves out of a WRAN service area and cannot receive a WRAN broadcast signal. IEEE 802.22 does not define the case where the customer premise equipment 102 cannot select a WRAN system and, in this case, the customer premise equipment cannot follow the subsequent steps of the flow and cannot couple to a network.